This invention relates to electrographic stylus recording and in particular to a method and apparatus for maintaining a controlled quantity of toner in the recording gap formed between an array of stylus electrodes and a receptor recording surface.
The broad field of electrographics may be regarded as involving creation of an image pattern through electrical means. Undoubtedly, the most widely used area of electrographics is that known as electrophotographics wherein the image pattern is created by directly subjecting a photosensitive recording member to a combination of electrical means and a light image. In electrostatic electrophotography, the image pattern exists as a pattern of electrostatic charges on the recording member.
Another broad area of electrographics, and the one to which this invention relates, does not necessarily rely upon the direct focusing of a light image on a photosensitive recording member, but rather forms the image pattern by electrical means alone.
Electrographic systems not utilizing direct light imaging find their major usage in oscillographic recording, computer printers, plotters, facsimile machines and the like. Some of the major systems in use can be characterized as spark recording (see U.S. Pat. Nos. 2,035,474 and 3,355,473), electrolytic recording (see U.S. Pat. No. 3,075,193), electrostatic stylus recording (see U.S. Pat. Nos. 2,932,690 and 2,932,548), and electrophoretic recording (see U.S. Pat. Nos. 3,121,375; 2,035,475 and 2,932,690).
In U.S. Pat. No. 3,816,840 to A. R. Kotz, an electrographic stylus recording process and apparatus is disclosed wherein electronically conductive toner at a recording region is presented between one electrode and the surface of a passive dielectric recording member which is in electronic contact with a second electrode. A portion of the toner is selectively deposited as image toner on the surface of the recording member in accordance with electrical potential signals applied across the two electrodes. U.S. Pat. No. 3,914,771 to Lunde uses the teachings of the Kotz patent, supra, and discloses a fixed cylindrical shell enclosing a rotatable roll containing a multiplicity of magnetic pole pieces with alternating north and south poles around the circumference of the roll. The magnet roll is rotated to cause an alternating magnetic force which varies in time, magnitude and polarity at the styli tips which are spaced from a dielectric recording member. Movement of the pole pieces cause magnetically attractable, electronically conductive toner presented to the shell to move around the shell through the gap presented between the styli tips and the recording member. Chains of toner particles regularly bridge the gap in response to the cyclic magnetic field produced by the rotating magnet roll to provide an electronically conductive path between the styli tips and dielectric recording member. Electronic pulses are applied to the styli and are synchronized with the rotation of each pole piece so as to be applied when a pole piece is positioned so that toner chains bridge the recording gap causing toner to be deposited on the recording member. The maximum practical printing speed for a given quality of printing (i.e., recorded dots/cm) of this rotating magnet machine is limited by the number of magnet pole pieces and the speed of rotation of the magnet roll. It is also recognized that since a good electronically conductive path is not maintained via the toner during a portion of the time that a magnetic pole piece of the magnet roll rotates past the styli tips, a truly continuous uniform line of toner in the direction of dielectric recording member motion cannot be deposited by a stylus using the teachings of the patent to Lunde.
U.S. Pat. No. 3,946,402 to Lunde recognizes the need for controlling the delivery of toner to a recording gap provided between a styli array and a dielectric recording member, but retains the same structure for the stylus array used with the stationary shell and rotatable roll of a plurality of magnetic pole pieces as disclosed in his U.S. Pat. No. 3,914,771, supra, and, therefore, requires synchronization of the applied electronic pulses with the position of the pole pieces. While control of delivery of the toner to the recording gap is obtained, a problem remains with respect to excess toner accumulation at the recording gap due to variations in the amount of recorded image that is formed. A varying supply of toner at the recording gap causes variation in size and placement of toner on the recording member at the stylus in response to a recording signal.